This patent application is co-pending with one related patent application entitled xe2x80x9cNON-INVASIVE METHOD OF DETERMINING DIASTOLIC INTRACRANIAL PRESSURExe2x80x9d (NASA Case No. LAR 16440-1), by the same inventors as this patent application.
1. Field of the Invention
This invention relates to determination of intracranial pressure. More specifically, the invention is a non-invasive method for determining the absolute intracranial pressure in a patient.
2. Description of the Related Art
The human brain and the spinal cord are immersed in a fluid called the cerebrospinal fluid (CSF) which is continuously generated and reabsorbed by the body. The CSF is contained in a membrane covering the inside of the skull and the spinal cord which terminates in a sack located at the sacrum. The brain and the membrane containing the CSF also contain blood vessels, which are in direct communication with the CSF and add to the total volume of the cerebrospinal system. The blood volume in these blood vessels varies rhythmically with the heartbeat thereby causing corresponding oscillations in the intracranial pressure (ICP). An accurate regulating process in the brain normally controls generation and reabsorption of CSF as well as the blood volume in the brain to maintain a constant ICP average value of about 40 mmHg. However, ICP changes when the regulating process is disturbed by, for example, tumors in the brain or trauma to the brain. Unfortunately, as little as 10 mmHg increase above average value in the ICP can cause insidious damage to the brain.
Given the above, monitoring ICP is of significant diagnostic and post-operative importance for patients with cranial injuries, pathologies or other conditions that may affect the pressure of the subarachnoidal fluid around the brain, and for patients who have undergone brain surgery. ICP has traditionally been measured and monitored by means of a pressure sensor inserted through the skull into the brain. Usually a hole is drilled in the skull and a catheter with a pressure sensor is inserted into the brain fluid. This known procedure, while simple and accurate is not suitable for long-term monitoring because an open wound must be maintained in the skull. Antibiotics are only partially effective in treating cranial infections so the pressure sensor typically can only be left in place for two weeks or less.
Long-term monitoring of ICP is currently achieved by implanting a pressure sensor and transmitter into the brain. The ICP is thereafter monitored by means of a receiver located outside the skull. However, this solution is not preferred because it includes the risks associated with implanting anything in the brain, and because of the problems of providing power to an implanted transmitter.
A variety of non-invasive systems and/or methods of measuring relative changes in ICP have been described in each of U.S. patent application Ser. Nos. 09/459,384, 09/493,044, 10/094,023, and 10/121,932. However, none of these provide for the measurement or determination of an absolute ICP. U.S. Pat. No. 5,617,873 discloses a method and system for monitoring absolute ICP, but requires the use of two known changes in the volume of CSF while recording corresponding changes in ICP by means of a calibrated measurement device.
Accordingly, it is an object of the present invention to provide a method of determining absolute ICP in a non-invasive fashion.
Another object of the present invention is to provide a method of determining absolute ICP that minimizes the number of procedures used.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is presented for determining absolute intracranial pressure (ICP) in a patient. In at least one embodiment, skull expansion of the patient is monitored as a function of time while changes in ICP in the patient are induced. Blood pressure of the patient is then measured at a time when skull expansion is approximately zero. The measured blood pressure at this time is indicative of a reference ICP value. A known change in ICP in the patient is caused after the time of zero skull expansion. A change in skull expansion associated with this known change in ICP is then measured. The absolute ICP is a function of the reference ICP value, the known change in ICP and the change in skull expansion associated with the known change in ICP.